The present invention discloses a method and apparatus for measuring a rotation rate of a downhole tool and, in particular, a method and apparatus for extending a measurement range of a gyroscope used to obtain a rotation rate of the drill tool.
In petroleum exploration, a borehole is formed in the earth in order to reach a hydrocarbon-bearing reservoir by rotating a drill bit at a bottom end of a drill string disposed in the borehole. Formation evaluations sensors on the drill string are used to obtain parameter measurements, e.g., resistivity, porosity, acoustic measurements, etc., which are then used to determine the nature of the formation surrounding the drill string and to help steer the drill string toward the reservoir. Formation parameter measurements are often obtained while the drill string is rotating and the tool face angle is varying rapidly. Therefore, for directionally-sensitive measurements, knowing the tool face angle of the drill string is useful. In many instances, the tool face angle can be calculated from an arctangent function of a two-axis magnetometer, with its X and Y axes perpendicular to the long (Z) axis of the drill string. This method is computationally intensive. The tool face angle can also be determined by integrating measurements of instantaneous rotation rate of the drill string. The instantaneous rotation rate is often measured using a gyroscope oriented such that its sensitive axis measures rotation around an axis parallel to the drill string. However, the gyroscope is reliable only for rotation rates within a certain range. When the drill string rotates at a rate outside of this range, as often happens during stick-slip conditions on the drill string, the gyroscope measurements may provide a poor measurement of the true rotation rate and produce errors in subsequent computations, such as a poor estimation of tool face angle, leading to a poor angular alignment of formation parameter measurements or to an erroreous correction for an antenna that is sensitive to rotation rate.